A typical fuel pumping system for an aero-engine comprises a low pressure (LP) pumping stage operable to draw fuel from a fuel tank, and supply the fuel at boosted pressure to the inlet of a high pressure (HP) pumping stage. The LP pumping stage may comprise a centrifugal impeller pump while the HP pumping stage may comprise a positive displacement gear pump having one or more pinion gear pairs.
The inter-stage flow between LP and HP pumping stages may be used to cool engine lubrication oil in a fuel/oil heat exchanger.
The journal bearings and gear elements of an HP pumping stage gear pump are typically lubricated by the fluid (aviation engine fuel) being pumped, due to the impracticalities of providing appropriate sealing.
FIG. 1 shows schematically a cross-section through part of a fuel pumping unit 30 which supplies fuel to the combustion equipment of a gas turbine aero-engine. The pumping unit 30 has an HP stage and an LP stage.
The HP stage is contained in an HP housing 31, and comprises a positive displacement pump in the form of two gear pumps: a secondary gear pump 32 and a primary gear pump. The secondary gear pump 32 includes a driver gear 34, and a driven gear 36, the teeth of which are meshed with one another, the gears 34, 36 being sandwiched between bearing blocks 38. The secondary gear pump 32 incorporates an input drive shaft 40 arranged to be driven by a drive output pad of an associated accessory gear box (AGB). The drive shaft 40 is arranged to drive the gear 34 for rotation, which in turn, by nature of the meshing of the gear teeth, drives the gear 36 for rotation. Rotation of the gears 34, 36 positively displaces fuel from an inlet side of the secondary gear pump 32 to an output side thereof, pressurising the fuel at the output side. The primary gear pump is driven through a secondary drive shaft 103 that connects driven gear 36 and a drive gear 100 of the primary gear pump. A driven gear (not shown in FIG. 1) of the primary gear pump meshes with the drive gear 100. An extension shaft 42, forming part of the drive shaft 40, is fixed into the drive gear 34, onto which is mounted an impeller 50 of a centrifugal pump forming part of the LP stage. In use, rotation of the input drive shaft 40 by the AGB drives the secondary gear pump 32, the primary gear pump, and the LP centrifugal pump.
The bearing blocks 38 include annular carbon face seals (CFSs), in which a carbon seal is urged against a harder surface by a spring element, in use the carbon seal and the harder surface rotating relative to each other with the interface between them forming the seal. Insertion and extraction of a CFS into the blind bore which contains the shaft of the respective gear 34, 36 can be highly dependent on the skill and experience of an operator. Typically, the spring element of the CFS has to be fully compressed upon location and then allowed to relax. Furthermore there may be CFS location pins that need to be correctly inserted in their location bores. For extraction, a conventional tool can be used in which two legs are inserted into the location pin bores, and the CFS is then “walked out” of its installation. However, the position of the pin bores can make it difficult to correctly insert the legs, resulting in failure of the tool to locate the CFS face and extract the CFS. The “walking out” extraction process can also result in a damaged CFS that needs to be replaced, and also potentially results in damage (scouring) to the pump casting.